1. Field of the Invention
The present invention generally relates to electric circuits especially adapted to assist soldering or welding machine tools, and more particularly to a welding control unit power supply for providing a substantially constant and powerful DC supply voltage derived from a high current welding power supply, and a welding control unit including the welding control unit power supply, for controlling the welding power supply provided by a welding machine (or a welding power source) to an electrode for welding a workpiece.
2. Description of the Related Art
It is a known problem with arc welding devices that welders are commonly subjected to potentially dangerous electric shocks as when working in a damp environment or when replacing a consumed electrode. Various attempts have been made in the art to increase the welder's safety, and from these attempts have evolved several manually or automatically operated welding control circuits for reducing electric shock hazards due to high open circuit voltages.
The welding control circuit of Simoneau et al. U.S. Pat. No. 4,514,615 is of particular interest in that it is relatively efficient, reliable and fast operating. The control circuit is based on detection of the impedance between the electrode and a workpiece so as to switch off the welding power supply to the electrode except when the detected impedance decreases under a predetermined threshold corresponding to a valid arc welding condition. The control circuit has however no provision with regard to the DC supply voltage source required for operation, nor with a fail-safe circuit to detect failure of the welding switch. In general, such a control circuit uses at least one fan for cooling purposes. However, to obtain an optimal output from the fan, its supply voltage must be highly stable.
Kroll U.S. Pat. No. 4,508,954 describes a welding system electrically supplied by the DC or AC welding power supply provided by a welding machine. The supply voltage required for operating a wire feed motor is derived from a bridge rectifier, acting as a crude voltage regulator. Several switches must be triggered during the system operation, and the supply voltage is subjected to significant variations in the welding power supply.
Corrigall et al. U.S. Pat. No. 4,450,340 describes a welding system equipped with a voltage reducing circuit which reduces the system output to a standby voltage lower than the full open circuit voltage. The voltage reducing circuit is provided with a fail-safe circuit whose operation is timed by an R-C circuit, so as to detect a voltage reduction failure after a predetermined time period.
In the prior art welding control circuit, no effective DC supply voltage source has been constructed to provide a substantially constant and powerful DC supply voltage derived from the high current welding power supply provided by a welding machine, voltage which must remain constant even upon occurrence of significant voltage variations in the welding power supply. Also, no prior art welding control circuit has been equipped with a failure detector which permanently detects a failure of the welding switch with respect to welding current and voltage conditions.